Green roof benefits for reducing flood risk at the catchment scale

Among the European countries, Italy is one of
the most susceptible to hydrogeological instability events
related to the risk of flooding in intensely populated basins,
especially inside the Po Valley. A clear example is the
Seveso river basin, whose territories are regularly stressed
several times per year by flooding events, which in recent
years have increased in frequency and intensity. However,
it is now impossible to intervene with structural projects
aimed at the lamination of flood events, even in modest
return periods because of excessive urbanization and continuous
residential and industrial expansion. Using extensive
and detailed green roof parameters, this study
modelled the hydrologic effect of three hypothetical roof
greening scenarios at the catchment scale (conversion of 5,
30, and 100 % impervious to green roofs). The modelling
of the green roof performances was performed using the
Environmental Protection Agency (EPA) Storm Water
Management Model (SWMM) and was calibrated over five
years of hourly discharge measurements at the closure
section of the basin. Hydrologic modelling demonstrated
that widespread green roof implementation significantly
reduces peak runoff rates and runoff volumes by up to 30
and 35 %, respectively, in the case of 100 % conversion.